When color matching must be done between devices having different color gamuts (e.g., upon outputting a color image displayed on a monitor using a printer), processing for compressing the color gamut is executed. Since the monitor and printer have different color gamuts, colors which can be represented by the monitor but cannot be represented by the printer are mapped to those which can be represented by the printer.
It is a common practice to compress the color gamut on a device-independent color space such as a CIELab space (to be referred to as an Lab space hereinafter). Especially, saturation compression uses a method such as linear compression (a of FIG. 1), linear compression based on zone division (b of FIG. 1), nonlinear compression (c of FIG. 1), or the like. For example, Japanese Patent Laid-Open No. 2003-283846 discloses a method of dividing into a region in which colors are maintained, and that in which colors are not maintained, and compressing saturation levels using different conversion lines for these regions.
However, when the linear compression (a of FIG. 1) is done, a low-saturation part especially suffers a saturation drop, and an output image with satisfactory tint cannot be obtained. This is because the human visual sense characteristics are sensitive to a change in saturation of a low-saturation region but are insensitive to a change in saturation of a high-saturation region. Also, the compression based on region division disclosed in Japanese Patent Laid-Open No. 2003-283846 produces discontinuous points and generates a pseudo contour in an output image like in the linear compression based on zone division (b of FIG. 1). Also, in order to implement the nonlinear compression (c in FIG. 1), a conversion curve must be obtained using complicated mathematical formulas or by connecting a plurality of points manually adjusted by the user via a spline curve, and the conversion curve cannot be automatically and uniquely obtained.